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默认图片

index.html
html
<body>
    <img id="img" />
</body>
<script>
const loadImage = url => new Promise((res, rej) => {
    const img = new Image()
    img.src = url;
    img.onload = () => res(url)
    img.onerror = err => rej(err)
})

loadImage('https://picsum.photos/300/200').then(url => {
    console.log(url)
    document.getElementById('img').src = url;
}).catch(err => {
    console.log(err)
    document.getElementById('img').src = 'https://picsum.photos/300/200';
})
</script>